Conventional motor vehicle shock absorbers typically incorporate a fluid-filled cylinder with a piston slidably disposed therein. Passageways are formed in the piston which restrict fluid flow between chambers on opposite sides of the piston. This serves to damp the motion of the piston relative to the cylinder. Representative prior art approaches are disclosed in U.S. Pat. Nos. 1,281,041, 2,410,992, 4,325,468, 4,474,271, 4,515,252, and 4,588,053. U.S. Pat. No. 2,393,559 discloses a somewhat similar approach utilized in connection with a timing device for use with dishwashing machines and the like.
Generally speaking, it is preferred that shock absorbers exhibit a low damping characteristic under normal conditions, thus providing a soft ride on relatively smooth road surfaces. On the other hand, the shock absorber should have the capacity to dampen severe shocks when the vehicle encounters adverse conditions such as potholes or raised obstructions in the roadway.
It is known to provide switchover from a low damping characteristic to a high damping characteristic in a shock absorber as a function of the position of the piston within the associated cylinder. That is, shock absorbers have been constructed in such a way as to provide a low damping characteristic when the piston is centrally located within the cylindar and a high damping characteristic when the piston approaches an end of the cylinder. Generally speaking, however, such arrangements are complex and expensive. For this reason they have not lent themselves to automobile use, but only to applications such as airplanes or industrial uses. In any event, shock absorbers of this type of construction are not always desirable or appropriate, as when, for example, a severe cavity or a particularly large raised obstruction is encountered in the roadway. Under such conditions it is possible that damage to the vehicle may occur by failure of the piston to exert a large damping force before it approaches the extremity of its stroke. At the very least, the occupants of the vehicle will be discomforted.
U.S. Pat. No. 4,610,332 illustrates a form of shock absorber piston valve which normally provides a low amount of damping but will switch into high damping mode when such is needed regardless of the positioning of the piston in its associated cylinder. This is accomplished by a relatively complex valving arrangement employing frusto-conical valve element in the piston to control the ports thereof. The valve element distorts under differing conditions to provide the desired effect. More specifically, the valve element provides a by-pass passage between one chamber of the shock absorber to another during a low velocity stroke of the piston but is sufficiently resilient to become seated and close off the by-pass passage in response to a relatively high velocity stroke. In this latter condition, all fluid is forced through the rebound port and the normal rebound valve to provide higher damping than when the by-pass passage is open. The reliability of such an arrangement which depends upon the repeated flexing of a relatively thin mechanical element is certainly problematical, and the fine tolerances which would have to be employed to ensure reasonable consistency of operation would make manufacture of such a unit an expensive matter. Another difficulty is the size of a piston incorporating this mechanism which, by necessity, is bulky and will unduly restrict the travel capacity of a shock absorber.